Machine installation control system

ABSTRACT

In a steam turbine driven pump installation, the steam flow to the turbine is controlled by a combined stop and throttle valve which includes a power operated actuator so as to vary the output of the pump, and a speed feed-back control is provided for the turbine comprising an indicator issuing a signal corresponding to the turbine speed, and a comparator comparing the speed signal with a desired value and delivering a control signal based on the comparison to a servo-device of the valve actuator for adjustment of the valve whereby turbine speed may be maintained constant. Additionally the installation includes an emergency control respondent to operating conditions in the installation e.g. turbine overspeed, oil pressure etc., the emergency control including a switch in the power supply to the valve actuator whereby in the event of the operating condition being unsatisfactory the switch is opened to close the steam valve.

[ 1 Jan. 7, 1975 MACHINE INSTALLATION CONTROL SYSTEM [75] Inventor: Forrest Thomson Randell, Glasgow,

Scotland [73] Assignee: Weir Pumps Limited, Glasglow,

Scotland 221 Filed: June8, 1973 211 App]. No.: 368,456

[30] Foreign Application Priority Data Primary Examiner-C. J. Husar Attorney, Agent, or Firm-Mason, Fenwick & Lawrence [57] ABSTRACT In a steam turbine driven pump installation, the steam flow to the turbine is controlled by a combined stop and throttle valve which includes a power operated actuator so as to vary the output of the pump, and a speed feed-back control is provided for the turbine comprising an indicator issuing a signal corresponding to the turbine speed, and a comparator comparing the speed signal with a desired value and delivering a control signal based on the comparison to a servo-device of the valve actuator for adjustment of the valve whereby turbine speed may be maintained constant. Additionally the installation includes an emergency control respondent to operating conditions in the installation e.g. turbine overspeed, oil pressure etc, the emergency control including a switch in the power supply to the valve actuator whereby in the event of the operating condition being unsatisfactory the switch is opened to close the steam valve.

10 Claims, 2 Drawing Figures V (SENSED SPEED SIGNAL) 19 (CONTROL SIGNAL] Vd.(DESIRED 17 SPEED n SIGNAL) MACHINE INSTALLATION CONTROL SYSTEM The present invention relates to a control arrangement for machine installations of the kind including a drive machine adapted to be operatively connected to a load, and particularly but not exclusively to an installation wherein the drive machine is constituted by a steam turbine. Such a machine installation is hereinafter referred to as a machine installation of the type aforesaid. The load may comprise a rotodynamic pump.

A previous control system for a steam turbine included an hydraulically actuated speed governor driven from a turbine oil pump auxiliary drive shaft and connected by mechanical linkage to a throttle valve in the steam supply line. Further, various trip mechanisms were mechanically linked to a separate stop valve in the steam supply line.

It is an object of the present invention to replace the stop valve and throttle valve of the previous arrangement by a combined stop, throttle and trip valve. This combined three-in-one valve serves the same func tion as the two separate valves did previously.

Further, in dispensing with the hydraulic governor, the need for an integral oil pump and oil sump is dispensed with, thus enabling the use of a modular lubri cating system. This system enables all the integral oil pumps and oil tanks required by each turbine to be replaced by one large oil tank, and one oil pump feeding a number of turbines at one time.

It is a further object of the present invention to provide a reduction in the number of components used to handle high temperature steam, and to provide an in herently safe control valve, and a more reliable control and tripping system. It is an object of the present invention to enable ease of maintenance due to a modular type construction, and to provide additional safety devices which will protect the turbine from abnormal operating conditions, whereas in the previous arrangement the turbine was unprotected.

According to the present invention a control arrangement for a machine installation of the type aforesaid comprises primary indicating means for sensing the drive speed of the drive machine or any other operating condition prevalent within the installation dependent on the drive speed, the primary indicating means generating signals proportional to the sensed speed or condition, and a comparator for comparing a sensed signal with a desired value and producing a control signal based on the comparison; speed regulating apparatus for the drive machine including an actuator supplied with power via a power supply line and operable to adjust the drive machine speed, and a servo-control device associated with the actuator and adapted to be set by said control signal to regulate the supply of power to the actuator and thereby the operation of the actuator whereby the drive speed or condition is maintained at the desired value; and secondary emergency indicating means including sensing means for sensing the drive machine speed and/or other operating conditions within the machine installation, and a switch in the said power supply line to the actuator operatively connected to the sensing means whereby when the speed or said other operating condition reaches an unsatisfactory value the switch is shifted to cause prompt stopping of the drive machine.

In a preferred embodiment of the present invention, the drive machine drives a cargo pump, for example in an oil tanker, the pump output being varied by variation of the drive machine speed, and the primary indicating means serves to control the speed of the drive machine.

The drive machine may be a fluid machine; and preferably the speed regulating apparatus comprises a throttle valve to control the flow of operating fluid to the drive machine, and the acutator of the throttle valve is operable by electric power, the switch being an electrical switch.

Preferably the primary indicating means, the comparator and the servo-control device are comprised by electronic devices whereby an electric control signal is delivered to said servo-control device to control the movement of the actuator.

In a preferred arrangement the actuator of the throttle valve includes an hydraulic device for control of the valve opening, and the speed regulating apparatus additionally includes an electric motor for supply of pressure fluid to the hydraulic device, electric power to the motor being supplied via said power supply line.

Preferably the primary indicating means is constituted by an electronic tachometer producing a signal proportional to the speed of the drive machine and linked to an electric transducer which is operatively as sociated with a drive shaft of the drive machine to measure the rotational speed of the shaft.

Preferably the secondary indicating means includes an electronic speed warning device arranged to deliver a signal when the speed or condition rises above a predetermined value, the switch being responsive to said signal to close the actuator.

Preferably the electronic speed warning device includes a signal issuing element.

It is desirable that the drive machine is stopped when there is a failure of the primary indicating means since otherwise the machine could operate out of control, and means may therefore be provided to close the actuator on failure of the primary indicating means. Preferably the primary indicating means are operatively connected to said signal issuing element whereby in the event of failure of said primary indicating means the actuator is moved to a closed position, and means are provided operable to over-ride said electronic speed warning device and permit an opening movement of the actuator at start-up of the machine installation.

The comparator preferably includes an electronic setting device for producing a signal proportional to the desired value of the sensed speed or condition, and said over-riding means may be embodied in the setting device to operate when the setting device is set at a predetermined value position.

Preferably a further mechanical actuable, speed warning device is provided to signal closure of the actuator when the speed of the drive machine increases above a predetermined value.

The secondary indicating means could include further sensing devices for sensing further conditions prevalent within the installation, for example:

1. Bearing oil pressure:

2. Machine temperature:

3. Machine vibration:

4. Machine wear: and

5. Rotational direction of the drive shaft.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows schematically the arrangement of a primary speed control feed-back circuit and an emergency control circuit, in a steam turbine driven pump installation: and

FIG. 2 shows schematically the nature and arrangement of the element of the circuits of FIG. 1.

Referring to the drawings, a machine installation includes a steam-driven turbine l, a driving shaft 2 of which is connected to a load 3 (not shown in FIG. 2), such as for example a rotodynamic pump say for the delivery of cargo oil to a shore installation. The steam flow 4 to the turbine 1 is regulated by a combined stop and throttle valve 5 including an hydraulic actuator 6 the pressure fluid of which is supplied by an electric motor driven machine 7, and the installation includes a primary feed-back electric circuit 8 to control the speed of the turbine drive shaft 2. An electro-servo device 9'including for example a potentiometer controls the power flow to the motor of machine 7 of the valve actuator 6 for setting of the valve 5, and this servo device 9 receives operating signals from the primary feedback circuit 8. The valve 5 is biased by spring means (not shown) into closure position for fail safe operation.

Referring to FIG. 2, the speed of the drive shaft 2 is sensed by a magnetic transducer 10 which is triggered by an annular series of six projections of the drive shaft periphery, say comprised by the edges of a hexagonal nut 11 or by the teeth of a toothed wheel, to produce an AC. signal v,- whose frequency is 6 times turbine shaft speed. The primary feed-back circuit 8 includes an electronic tachometer 12 in the form of a module to convenience removal and replacement, the module housing an amplifier 13 receiving the A.C. signal v,-, and a pulse counter 14 to which an output signal v from the amplifier 13 is passed, the pulse counter 14 providing an output dc. voltage V, directly proportional to the turbine shaft speed. Additionally an indicator 15 is provided actuated by the output dc. voltage signal V, for visual indication of the speed.

The dc. output signal V, from the tachometer 12 is fed to an electronic comparator 16 wherein the signal is compared with a reference (or input) voltage V,,, proportional to the desired turbine shaft speed, the reference voltage V being produced in a voltage setting device 17, for example a potentiometer.

A difference or error voltage V, is produced by the comparator l6, and this error voltage is amplified and changed in polarity so that a negative voltage is produced when the shaft speed exceeds the desired value d and a positive voltage is produced when the shaft speed is below the desired value d. The output or error voltage V from the comparator 16 is fed to a voltageto-current converter 18, so that a current control signal I, is delivered to the electro-servo device 9 of the motor of mechanism 7. it is arranged that a full negative output voltage from the comparator 16 produces a 4 milliamp output from the converter 18, while a full positive voltage produces a milliamp current signal 1 and a negative current signal will cause closure of the steam valve, full closure occurring with a 4 milliamp negative signal, while a positive current signal will open the steam valve, the valve being fully opened by a 20 milliamp positive signal.

The comparator 16, voltage setting device 17 and converter 18 are in the form ofa module 19 to facilitate installation and replacement. The proportional band i.e. the change of turbine shaft speed required to cause the steam valve to move from a closed to a fully open position can be varied by changing the voltage gain of the comparator l6.

It is essential that the speed of the turbine is prevented from increasing beyond a predetermined upper value, otherwise damage to the machine could occur, and to this end the machine installation is provided with an electrical overspeed control circuit 20 (FIGS. 1 and 2) to close the steam valve 5 when the turbine speed increases beyond the predetermined value. The circuit includes an amplifier 21 additionally receiving the ac voltage signal v from the magnetic transducer 10, and the amplifier output voltage signal v from the amplifier 21 is fed to an electrical filter 22 having a characteristic such that up to a predetermined input, the output v is of constant value, but when'the input increases beyond the predetermined value, the output V falls very sharply. The filter 22 is operatively connected to a trip relay 23 whereby when the filter output v falls sharply a trigger circuit of the relay 23 is actuated to deenergise the relay 23. The electrical motor of mechanism 7 of the steam valves hydraulic actuator 6 is supplied with electric power via line 24 and an electrical contactor switch 25 is provided in the power line 24 to control the electic power supply to the motor 7. The switch 25 is energised through the trip relay 23 to maintain the fluid valve 5 in the open position. How ever, when the trip relay is de-energised, the contactor switch 25 opens to cut-off power to the motor 7 and the steam valve 5 is consequently urged rapidly into a closure position by the spring means.

The amplifier 21, filter 22 and trip relay 23 of the overspeed circuit are advantageously arranged as a module 26. It is essential that in the event of failure of the electrical tachometer 12, the steam valve 5 is signalled to close, otherwise the speed of the turbine may rise dangerously due to the turbine speed being out of control. Thus, the tachometer output is connected additionally into the trip relay of the overspeed circuit via connecting line 27, whereby in the event of tachometer 12 failure the relay is de-energised to open the contactor 25 and cause prompt closure of the steam valve 5. However, since the tachometer 12 is inoperative, at start up, with the result that the contactor 25 is signalled to open through the trip relay 23 of the over speed circuit, it is necessary to over-ride the overspeed circuit at start up, to permit starting of the turbine. Therefore, a microswitch (not shown) is associated with the setting device (17) and is arranged to close when the setting device 17 is adjusted to a start position, i.e. such as to give predetermined opening of the steam valve 5, a biasing voltage consequently being passed via line 28 of the trip relay 23 from the microswitch to the output to over-ride the output voltage from the overspeed control and maintain the contactor 25 in a closed position.

Referring to FIG. 1, the machine installation could include further secondary sensing devices monitoring further conditions prevalent within the installation and would be included in the circuit 20. For example, a sensor 30 (FIG. 2) could be provided to monitor the lubrieating oil pressure at the turbine bearings and/or at the bearings of other machines, for example a pump in the installation, and the oil pressure sensor 30 would be connected to the contactor switch 25 so that the steam valve 5 is closed to stop the turbine 1 when the oil pres sure level falls below a desired value. Further sensing devices (e.g. item 31 in FIG. 2) could monitor for example:

a. Machine temperature:

b. Machine vibration:

c. Wear in the machine installation:

d. Pressure condition at farther locations in the installation: and

e. Anti-rotation of the turbine shaft (such as could occur on inadvertent return of oil from the shore installation).

Each sensing device 31 would be connected to the contactor switch 25 similarly as for the pressure sensing device 30; and each sensing device can advantageously be a modular form to facilitate location of the device in the installation.

Additionally, since the electrical operspeed control is rendered inoperative at start-up of the turbine, a further, mechanically actuable, overspeed control 32 is driven from the turbine shaft 2 as a safety precaution and is connected into the contactor switch 25 for stoppage of the turbine 1 when turbine speed increases dangerously; this further overspeed control 32 also provides a safety factor against tachometer 12 failure at start-up. The start 33a and stop 33b controls of the turbine operate through the contactor switch 25. The speed control module 19 also includes a speed limit control 34 which is coupled to the voltage-to-current converter 18 and limits the run up speed of the turbine l, at turbine start up, and which is additionally coupled to the voltage setting device 17 so that the control 34 is cut out when the setting device 17 is adjusted from the start setting. The speed limit control 34 could for example include a diode (not shown) which is brought into the speed control circuit by the closure of a second micro-switch (not shown) associated with the voltage setting device 17, which micro-switch is closed when the setting device 17 is adjusted to the start position, and the speed limit control 34 limits the control signal 1,. to the servo'device 9 of the steam valve motor 7.

To start the turbine 1, the voltage setting device 17 is firstly adjusted to its start setting so that nonindication by the tachometer l2 alone cannot cause opening of the contactor switch 25 through the electrical overspeed circuit and the turbine start control is actuated to open the valve 5 whereby the steam valve opens sufficiently to allow the turbine to run up to a given speed. It can be arranged that the speed limit control 34 limits the turbine start up speed to say 2000 rpm, but any other desired speed value could be used. With the turbine started, the voltage setting device 17 is adjusted slowly to the desired running speed of the turbine, the micro-switches of the setting device consequently being actuated to energise the electrical overspeed control 20 and to cut out the speed limit control 34, and the primary feed-back control 8 will operate to maintain the turbine speed at the desired running value. Any small increase in turbine speed from the set running speed will cause the steam valve 5 to move towards the closed position and any decrease in turbine speed will cause opening of the steam valve 5.

If the turbine speed should rise above the set value, or if any of the other sensed conditions within the installation, such as for example the lubricating oil pres sure or tachometer operation, should fail or vary dangerously from the set value, the appropriate sensing device is triggered to open the contactor 25 and so shut the steam valve 5 to stop the turbine 1. It is desirably arranged that when the electrical overspeed control 20 is triggered, the control 20 will not re-energise when the turbine speed falls to the set running speed, since otherwise it would be possible to restart the turbine with the voltage setting device 17 of the speed control in any position and this would probably result in further triggering of the overspeed control 20 when the turbine has re-started.

The turbine 1 may drive a rotodynamic pump, and sensing devices as described above could be used for example to sense pump suction and/or pump discharge pressure, and to control the turbine speed, when these pressure conditions vary undesirably. In the Applicants US. Pat. No. 3,797,966, a control system is described for controlling the pressure conditions at pump discharge in a machine installation (particularly for the supply of boiler feed) and it will be understood that the above described primary feed-back control circuit could likewise be adapted to vary the turbine speed in dependence on pump discharge pressure requirements, rather than serving to maintain a constant turbine speed. Cutting off the power supply to the valve actuator has the considerable advantage, particularly in overspeed control, of permitting very rapid closure of the steam valve.

Modifications are of course possible in the above de scribed control apparatus. For example, in the case of the sensor (e) above for anti-rotation of the turbine shaft, this sensor could be connected into a turbine run ning control so as to initiate running of the pump for a short duration when inadvertent oil return flow occurs from the shore installation to the pump.

I claim: 1. A control arrangement for a machine installation of the type including a drive machine for connection to a load, and particularly but not exclusively to an installation wherein the drive machine is. constituted by a steam turbine, the control arrangement comprising in combination:

a. primary indicating means for sensing any of the drive speed of the drive machine and other operat ing condition prevalent within the installation dependent on the drive speed, the primary indicating means generating signals proportional to the sensed condition; b. a signal generating device settable to produce a signal proportional to the desired value for said sensed condition; c. a comparator including an output for a control signal; d. line means delivering the produced signal from the primary indicating means and the desired value signal from the signal generating device to said com parator whereat the signals are compared and an output control signal produced based on the comparison; e. speed regulating apparatus for the drive machine including i. an actuator having a motor operable for adjustment of the drive machine speed;

ii. a power supply line connectible to a power source for supplying power to said actuator motor; and

iii. a servo-control device for regulating the power supply to said actuator motor and thereby the operation of the actuator for adjustment of the drive machine speed;

f. conduit means for delivering the control signal from the comparator output to the servo-control device for setting of the device so that the desired value for the measured operating condition may be maintained; and

g. secondary emergency indicating means including:

i. sensing means for sensing any'of the drive machine speed and other operating condition within the machine installation,

ii. a switch in said power supply line to the actuator motor and movable to an open position to cut-off the supply of power to said actuator motor for prompt stoppage of the drive machine, and

iii. connecting means operatively connecting the sensing means to said switch whereby when the operating condition sensed by said sensing means reaches an unsatisfactory value the switch is moved to said open position to stop the drive machine.

2. A control arrangement as claimed in claim 1, wherein the primary indicating means senses the drive speed of the drive machine. A

3. A control arrangement as claimed in claim 1, wherein the drive machine is a fluid machine, and the speed regulating apparatus comprises a throttle valve to control the flow of operating fluid to the drive machine, and the actuator of the throttle valve is operable by electric power, the switch beingan electrical switch.

4. A control arrangement as claimed in claim 1, wherein the primary indicating means, the comparator and the servo-control device are comprised by electronic devices whereby an electric control signal is delivered to said servo-control device to control the movement of the actuator.

5. A control arrangement as claimed in claim 3, wherein the actuator of the throttle valve includes a hydraulic device for control of the valve opening, and the speed regulating apparatus additionally includes an electric motor driven machine for supply of pressure fluid to the hydraulic device, electric power to the motor being supplied via said power supply line.

6. A control arrangement as claimed in claim 4, wherein the secondary emergency indicating means includes an electronic warning device having an electrical filter having a characteristic such that up to a predetermined electric input to the filter the filter output is of constant value, but when the input increases beyond the predetermined value the filter output falls sharply, the filter input corresponding to the measured condition, and the filter is operatively connected to a trip relay whereby when the filter output falls sharply when the speed of the drive machine increases beyond the predetermined value, the relay is actuated to open the switch.

7. A control arrangement for a machine installation of the type including a drive machine for connection to a load, and particularly but not exclusively to an installation wherein the drive machine is constituted by a steam turbine, the control arrangement comprising:

a. primary indicating means for sensing any of the drive speed of the drive machine and other operating condition prevalent within the installation dependent on the drive speed, the primary indicating means generating signals proportional to the sensed condition;

b. a signal generating device settable to produce a signal proportion to the desired value for said sensed condition;

c. a comparator including an output for a control signal;

d. line means delivering the produced signal from the primary indicating means and the desired value signal from the signal generating device to said comparator whereat the signals are compared and an output control signal produced based on the comparison;

e. speed regulating apparatus for the drive machine including:

i. an actuator having a motor operable for adjustment of the drive machine speed,

ii. a power supply line connectible to a power source for supplying power to said actuator motor; and

iii. a servo-control device for regulating the power supply to said actuator motor and thereby the operation of the actuator for adjustment of the drive machine speed;

f. conduit means for delivering the control signal from the comparator output to the servo-control device for setting of the device so that the desired value for the measured operating condition may be maintained;

g. secondary emergency indicating means including:

i. sensing means for sensing any of the drive machine speed and other operating condition within the machine installation,

ii. a switch in said power supply line to the actuator motor and movable to an open position to cut-off the supply of power to said actuator motor for prompt stoppage of the drive machine, and

iii. connecting means operatively connecting the sensing means to said switch whereby when the operating condition sensed by said sensing means reaches an unsatisfactory value the switch is moved to said open position to stop the drive machine; and

h. a switch closing element for said switch of the secondary emergency indicating means;

i. a line connecting said switch closing element to said primary indicating means for the delivery of primary signals to said switch closing element, the switch closing element being actuable by said signals generated by said primary indicating means to close the switch;

j. a drive machine start-up setting device including an element movable to a first position to permit startup of the drive machine and adjustable to a further position for normal running of the machine; and

k. over-ride means for said switch closing element operatively connected to said drive machine startup setting device so that an over-ride signal is produced when the setting device element is moved to said first position for machine start-up, which override signal is eliminated when the setting device element is adjusted to said further position for normal running of the drive machine, the over-ride sig nal serving for switch closure at drive machine start-up in the absence of a primary signal from the primary indicating means.

8. A control arrangement as claimed in claim 7, wherein the primary indicating means is constituted by an electronic tachometer producing an electrical signal proportional to the speed of the drive machine and linked to an electric transducer which is operatively associated with a drive shaft of the drive machine to measure the rotational speed of the shaft, the secondary indicating means includes an electronic device measuring an operating condition in the installation and delivering a warning signal when the said condition rises above a predetermined value, the switch being responsive to said warning signal to close the actuator, said electronic device of the secondary indicating means including a relay element operatively connected to both the primary indicating means and the switch of the secondary emergency control whereby in the event of failure of the tachometer the relay element is actuated in the absence of a signal from the tachometer to open the switch and thereby close the actuator, and said override means is comprised by an electrical device operable by said drive machine start-up setting device to pass an electrical signal to said switch to permit an opening movement of the actuator at start-up of the machine installation.

9. A control arrangement as claimed in claim 8, wherein said signal generating device includes an electronic setting device for producing a signal proportional to the desired value of the sensed condition, and said over-ride means is included in the setting device to operate when the setting device is set at a predetermined value position.

10. A control arrangement as claimed in claim 7, wherein the primary indicating means includes a speed limiting device operable at start-up of the drive machine, when said over-ride means are operable, to limit the build up of speed by the machine. 

1. A control arrangement for a machine installation of the type including a drive machine for connection to a load, and particularly but not exclusively to an installation wherein the drive machine is constituted by a steam turbine, the control arrangement comprising in combination: a. primary indicating means for sensing any of the drive speed of the drive machine and other operating condition prevalent within the installation dependent on the drive speed, the primary indicating means generating signals proportional to the sensed condition; b. a signal generating device settable to produce a signal proportional to the desired value for said sensed condition; c. a comparator including an output for a control signal; d. line means delivering the produced signal from the primary indicating means and the desired value signal from the signal generating device to said comparator whereat the signals are compared and an output control signal produced based on the comparison; e. speed regulating apparatus for the drive machine including i. an actuator having a motor operable for adjustment of the drive machine speed; ii. a power supply line connectible to a power source for supplying power to said actuator motor; and iii. a servo-control device for regulating the power supply to said actuator motor and thereby the operation of the actuator for adjustment of the drive machine speed; f. conduit means for delivering the control signal from the comparator output to the servo-control device for setting of the device so that the desired value for the measured operating condition may be maintained; and g. secondary emergency indicating means including: i. sensing means for sensing any of the drive machine speed and other operating condition within the machine installation, ii. a switch in said power supply line to the actuator motor and movable to an open position to cut-off the supply of power to said actuator motor for prompt stoppage of the drive machine, and iii. connecting means operatively connecting the sensing means to said switch whereby when the operating condition sensed by said sensing means reaches an unsatisfactory value the switch is moved to said open position to stop the drive machine.
 2. A control arrangement as claimed in claim 1, wherein the primary indicating means senses the drive speed of the drive machine.
 3. A control arrangement as claimed in claim 1, wherein the drive machine is a fluid machine, and the speed regulating apparatus comprises a throttle valve to control the flow of operating fluid to the drive machine, and the actuator of the throttle valve is operable by electric power, the switch being an electrical switch.
 4. A control arrangement as claimed in claim 1, wherein the primary indicating means, the comparator and the servo-control device are comprised by electronic devices whereby an electric control signal is delivered to said servo-control device to control the movement of the actuator.
 5. A control arrangement as claimed in claim 3, wherein the actuator of the throttle valve includes a hydraulic device for control of the valve opening, and the speed regulating apparatus additionally includes an electric motor driven machine for supply of pressure fluid to the hydraulic device, electric power to the motor being supplied via said power supply line.
 6. A control arrangement as claimed in claim 4, wherein the secondary emergEncy indicating means includes an electronic warning device having an electrical filter having a characteristic such that up to a predetermined electric input to the filter the filter output is of constant value, but when the input increases beyond the predetermined value the filter output falls sharply, the filter input corresponding to the measured condition, and the filter is operatively connected to a trip relay whereby when the filter output falls sharply when the speed of the drive machine increases beyond the predetermined value, the relay is actuated to open the switch.
 7. A control arrangement for a machine installation of the type including a drive machine for connection to a load, and particularly but not exclusively to an installation wherein the drive machine is constituted by a steam turbine, the control arrangement comprising: a. primary indicating means for sensing any of the drive speed of the drive machine and other operating condition prevalent within the installation dependent on the drive speed, the primary indicating means generating signals proportional to the sensed condition; b. a signal generating device settable to produce a signal proportion to the desired value for said sensed condition; c. a comparator including an output for a control signal; d. line means delivering the produced signal from the primary indicating means and the desired value signal from the signal generating device to said comparator whereat the signals are compared and an output control signal produced based on the comparison; e. speed regulating apparatus for the drive machine including: i. an actuator having a motor operable for adjustment of the drive machine speed, ii. a power supply line connectible to a power source for supplying power to said actuator motor; and iii. a servo-control device for regulating the power supply to said actuator motor and thereby the operation of the actuator for adjustment of the drive machine speed; f. conduit means for delivering the control signal from the comparator output to the servo-control device for setting of the device so that the desired value for the measured operating condition may be maintained; g. secondary emergency indicating means including: i. sensing means for sensing any of the drive machine speed and other operating condition within the machine installation, ii. a switch in said power supply line to the actuator motor and movable to an open position to cut-off the supply of power to said actuator motor for prompt stoppage of the drive machine, and iii. connecting means operatively connecting the sensing means to said switch whereby when the operating condition sensed by said sensing means reaches an unsatisfactory value the switch is moved to said open position to stop the drive machine; and h. a switch closing element for said switch of the secondary emergency indicating means; i. a line connecting said switch closing element to said primary indicating means for the delivery of primary signals to said switch closing element, the switch closing element being actuable by said signals generated by said primary indicating means to close the switch; j. a drive machine start-up setting device including an element movable to a first position to permit start-up of the drive machine and adjustable to a further position for normal running of the machine; and k. over-ride means for said switch closing element operatively connected to said drive machine start-up setting device so that an over-ride signal is produced when the setting device element is moved to said first position for machine start-up, which over-ride signal is eliminated when the setting device element is adjusted to said further position for normal running of the drive machine, the over-ride signal serving for switch closure at drive machine start-up in the absence of a primary signal from the primary indicating means.
 8. A control arrangement as claimed in claim 7, wherein the primAry indicating means is constituted by an electronic tachometer producing an electrical signal proportional to the speed of the drive machine and linked to an electric transducer which is operatively associated with a drive shaft of the drive machine to measure the rotational speed of the shaft, the secondary indicating means includes an electronic device measuring an operating condition in the installation and delivering a warning signal when the said condition rises above a predetermined value, the switch being responsive to said warning signal to close the actuator, said electronic device of the secondary indicating means including a relay element operatively connected to both the primary indicating means and the switch of the secondary emergency control whereby in the event of failure of the tachometer the relay element is actuated in the absence of a signal from the tachometer to open the switch and thereby close the actuator, and said over-ride means is comprised by an electrical device operable by said drive machine start-up setting device to pass an electrical signal to said switch to permit an opening movement of the actuator at start-up of the machine installation.
 9. A control arrangement as claimed in claim 8, wherein said signal generating device includes an electronic setting device for producing a signal proportional to the desired value of the sensed condition, and said over-ride means is included in the setting device to operate when the setting device is set at a predetermined value position.
 10. A control arrangement as claimed in claim 7, wherein the primary indicating means includes a speed limiting device operable at start-up of the drive machine, when said over-ride means are operable, to limit the build up of speed by the machine. 